Small hole ecm drilling with controlled current

ABSTRACT

Electrochemical machining of small deep holes (several hundredths of an inch in diameter and less) is achieved at higher feed rates utilizing a constant current source and constant feed rate, allowing the voltage and power to vary with depth. Small holes with profiled cross section are achieved by programming the controlled current for different current/feed rate ratios as a function of hole depth.

United States Patent 1 Joslin [4 1 Feb. 19, 1974 SMALL HOLE ECM DRILLINGWITH CONTROLLED CURRENT Inventor: Frederick R. Joslin, Lebanon, Conn.

United Aircraft Corporation, East Hartford, Conn.

Jan. 10, 1972 Assignee:

Filed:

Appl. No.:

US. Cl. 204/129.55, 204/224 M Int. Cl B23p l/02, B23p l/l2, B23p l/16Field of Search 204/224 M, 129.25, 129.55

[56] References Cited UNITED STATES PATENTS 12/1971 Koire et a1. 204/224M /1962 Williams 204/224 M 6/1968 Shibasaki 204/l29.75

OTHER PUBLICATIONS et a1. Electrochemical Machining, 1968 De Barr Amer.Elsenier Pub. Co., NY, NY, pages -47, -72, -82 and 191-192.

Uhlir, Jr., The Review of Scientific Instruments Vol. 26, No. 10 October1955 pp. 965-968.

Primary Examiner-F. C. Edmundson Attorney, Agent, or FirmMelvin PearsonWilliams [5 7] ABSTRACT 1 Claim, 7 Drawing figures SMALL HOLE ECMDRILLING WITII CONTROLLED CURRENT BACKGROUND OF THE INVENTION l. Fieldof Invention This invention relates to electrochemical machining, (ECM),and more particularly to improved ECM drilling of small holes.

2. Description of the Prior Art As is known in the ECM art, the presenceof an electrolyte in a suitably confined gap between an advancing hollowcathode tool with outer surfaces electrically insulated except at itsfrontal (gap) area and a workpiece anode, with a suitable voltageimpressed across the gap, comprises an electrolytic cell in which anelectrochemical process will cause controlled erosion of the metal ofthe workpiece. Employing a suitable slightly alkaline saline electrolyteproduces an insoluble metal hydroxide sludge which is flushed away andhydrogen is evolved at the cathode.

In the ECM drilling of holes, a thin hollow cathode conducts theelectrolyte to the work area through its central bore, and the effluentelectrolyte flows away from the work area in the annular space createdbetween the surface of the hole being drilled and the hollow cathode.Electrical power, equal to the product of the voltage across the gaptimes the current through the gap, accomplishes the useful work of metalremoval and also creates heat in the work area, which warms theelectrolyte. The effluent electrolyte exiting from the drilled holewarms the incoming electrolyte through the walls of the hollow-cathode,in a heat exchange type of action. In the drilling of holes on the orderof one hundredth of an inch in diameter, the rate of flow of electrolyteis extremely small, being on the order of one drop every 5 seconds.Because of the heat exchange action and the low flow rate, thetemperature of electrolyte in the gap can rise considerably, and tendsto become increasingly higher as the hole depth increases.

In ECM processes known to the prior art, a constant voltage is employedacrossthe gap. As the temperature increases, the conductivity of theelectrolyte increases so that the current in the gap increases. With aconstant voltage, an increase in current means an increase in the powergenerated in the gap, which power is taken up in the form of heat withinthe electrolyte, causing a further increase in the temperature. If thetemperature becomes sufficiently high, the electrolyte will boil,causing vapor pockets in the gap; the vapor pockets insulate the anodefrom the cathode and thus preclude the electrochemical reaction fromoccurring; and, as the hollow cathode continues to advance, withouterosion of the workpiece, the gap becomes smaller. If not corrected,this process may culminate in an unintentional contact between thecathode tip and the workpiece, which is commonly referred to as aspark-out.

Devices known to the prior art have obviated the situation by utilizingreduced feed rates. As an example, prior art feed rates for ECMdrillingof small holes are typically limited to below one tenth of an inch perminute. For a given hole diameter, the rate of metal re} moval (feedrate) is proportional to the current. The achievement of high feed ratesrequires the ability to utilize higher currents without spark-outs.

SUMMARY OF INVENTION The object of the present invention is to provideimproved ECM drilling of small holes.

According to the present invention, holes less than three hundredths ofan inch in diameter and with a ratio of depth to diameter in excess often, are drilled utilizing an electrochemical machining process in whichthe current is completely controlled. In accordance with one embodimentof the invention, drilling of substantially uniform straight holes isachieved employing a constant current in an ECM process. In accordancewith another embodiment of the invention, drilling of contoured holes isaccomplished by programming the current to feed rate ratio as a functionof hole depth.

The present invention provides an order of magnitude increase in therate at which small holes can be drilled using ECM processes. Theinvention also permits the drilling of holes having desired contours,which are not otherwise readily obtainable. The invention provides avery practical method of producing small holes in ordinary metals aswell as super alloys.

In drilling holes employing the process of the present invention (withthe voltage being a dependent variable resulting from fixed current andother parameters), as the depth of the hole progresses, the heat buildsup and the conductivity of the electrolyte increases; the voltagenecessarily decreases, so the thermal power input to the electrolytegoes down. Thus there is a tendency for the process to thermally limititself. This in turn necessarily results in mitigating the preheatingeffect which the effluent electrolyte has on the incoming electrolyte.Metal removal rate is a function of current which is held constant.Therefore the hole diameter remains essentially constant with a fixedfeed rate despite increases of electrolyte temperature in the gapproducing corresponding decreases in gap voltage. In a typical case thegap electrolyte has exhibited alinear negative temperature coefficientof resistance of 0.33%/F from 1 10 to .170F.

Other objects, features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof preferred embodiments thereof, as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified schematicillustration of electrochemical machining apparatus which may beemployed in the practice of the present invention;

FIGS. 2, 4 and 6 are sectioned, side elevations of contoured holes; andv I FIGS. 3, 5 and 7 are current versus depth plots relatingrespectively to the contours of FIGS. 2, 4 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT 26. The feed mechanism 26includes a ram 28 on which the collet 24 or other attaching means issuitably disposed. The ram 28 may be journaled in a precision ball guideassembly comprising a plurality of balls (such as the balls 30) confinedby a suitable plate (such as the plate 32). In fact, the entire surfaceof the ram 28 may preferably be guided by additional balls and plates(not shown). The ram 28 may be advanced by a rack 34 and pinion 36driven by a shaft 38 connected to a synchronous motor 40. Alternativelya precision ground ball screw or other suitable precise feed mechanismmay be utilized, as is known in the art. A controlled current DC source42 is connected by a suitable lead 44 to the workpiece 10, and by asuitable lead 46 to the collet 24, thereby to impress a voltage betweenthe hollow cathode 14 and the workpiece 10.

In operation, the electrolyte under a suitable pressure is driventhrough the hollow cathode 14 and out the end thereof in the vicinity ofthe gap formed between the end of the hollow cathode 14 and theworkpiece metal in an area having a diameter slightly larger than thediameter of the hollow cathode 14 is eroded by electrochemical action,the metal forming a metallic hydroxide sludge which is flushed out ofthe hole as the effluent electrolyte passes upwardly in an annularpassage formed between the surface of the hole '12 andthe hollow cathode14. The effluent electrolyte is collected in a suitable sump (notshown), as is known in the art.

In accordance with one embodiment of the invention, substantiallyuniform deep holesare drilled at a relatively high feed rate.

As an example, consider a beryllium copper hollow cathode 14 having a 5mil ID and a ten mil OD covered with a third of a mil parylene Cinsulation, used in drilling a high temperature nickel base alloy. Anelectrolyte consisting of a saline solution (two pounds per galdrilled.The initial volta e was approximately ten volts and the voltage decayetoabout 6% volts as the drilling proceeded. In a similar example, a holewas drilled at a rate of 0.3 inches per minute utilizing 400 rnilliampsconstant current.

The examples just described, utilizing constant current, produce asubstantially uniform hole at a relatively high rate of feed. Dependingon the material being worked, and particularly in cases where higherfeed rates are used, it is possible that constant current ECM drillingmay result in a hole having a slight taper (that is an increase indiameter as a function of depth of approximately 0.001 per inch). Inaccordance with another aspect of the invention, this can be eliminatedby programming the power supply as a function of depth to decrease thecurrent slightly as the drilling depth increases. This is illustrated inFIG. 1 by a depth sensor 48 which is connected by a lead 50 to thecontrolled current DC source 42. The depth sensor 48 may comprise avariable resistance having a wiper arm at tached to the ram 28, or anyother suitable position sensor capable of response to the position ofthe ram 28, the collet 24, or the hollow cathode 14. In accordance withteachings known to the art, a variable resistance or voltage derivedtherefrom may be utilized so as to adjust the current setting of thecontrol current DC source 42 as a function of depth.

In accordance with another aspect of the invention, the controlledcurrent DC source 42 may be programmed to provide current (I) as alinear or nonlinear function 'of depth (D) so as to provide any desiredhole contour as illustrated in FIGS. 2-7. A jug-like shape is achievedas seen in FIG. 2 by first decreasing the current and then increasing itas illustrated in FIG. 3. A wasp-waist configuration as seen in FIG. 4is provided by a concave upward current curve as shown in FIG. 5. In ahole with an increasing diameter (suitable for use with cast ordeformable rivets) as seen in FIG. 6, may be achieved by providing anincrease in current with depth as illustrated in FIG. 7.

Although the invention has been shown and described with respect topreferred embodiments thereof, it should be understoodby those skilledin the art that various changes and omissions in the form and detailthereof may be made therein without departing from the spirit and thescope of the invention.

Having thus described typical embodiments of my invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:

1. In the process of drilling a small hole, having a diameter less'thanthirty mils and and a ratio of depth to diameter in excess of ten, in ametallic workpiece, the steps of:

positioning an exteranlly insulated hollow electrode,

having an'outside diameter of less than 30 mils, adjacent a surface of aworkpiece in which said small hole is to be drilled; impressing avoltage of between 6.5 and ten volts from a constant current DC sourceof between 275 and 400 milliamperes between said workpiece and saidelectrode, poled to cause said electrode to act as a cathode andsaid'workpiece to act as an anode;

flowing a saline electrolyte through said electrode at a pressure on theorder of 50 PSI; and advancing'said electrodein a direction toward saidworkpiece at a rate of between 0.18 and 0.30

l inches per minute.

